1. Technical Field This invention relates to a sensing optical fiber and a sensor system thereof, which both have the. functions of an optical transmission line and a sensor device and serve to detect various types of information by virtue of the light transmission loss. More particularly, this invention relates to an optical system having improved sensitivity of detecting information by virtue of the light transmission loss and discriminating the type of the detected information.
2. Description of Related Art
An OTDR (optical time domain reflectometry) method for detecting information of light transmitted through an optical fiber is known in the art. This detection method serves to measure the information of the light transmitted through a light transmission line at an arbitrary position of the light transmission line in real time, and is carried out by letting light pulses of a laser beam or the like into an optical fiber, causing the incident light pulses to turn back halfway as back-scattered light toward the incident side (backward side), and effecting time-resolved measurement of the back-scattered light.
A known technique, using the detection method, includes an optical fiber that serves as the light transmission line is connected to an OTDR device having the function of a linear sensor, so that information as to an ambient temperature around the light transmission line is detected by measuring Raman scattered light having high temperature dependency in the back-scattered light or information as to distortion of the optical fiber transmission line is detected by measuring Rayleigh scattered light having high scattering coefficient in the back-scattered light.
The Raman scattered light is feeble in scattering coefficient, and the incident light for the Raman scattered light differs in wavelength from the scattered light of the same. Therefore, the OTDR device, which serves as a measuring instrument, and its attachments tend to be complicated and expensive. Under the circumstances, a need has been felt for a generalized technique capable of using the Rayleigh scattered light.
The aforementioned prior art entails a disadvantage such that the information detecting device making use of the Rayleigh scattered light is lower in sensitivity to temperature, distortion or the like of. the optical fiber transmission line, and thus, it is difficult in practical application.
The first technical object of the present invention is to provide a sensing optical fiber capable of increasing the sensitivity of detecting information by an OTDR method making use of Rayleigh scattered light.
The second technical object of the present invention is to provide a sensor system capable. of increasing the sensitivity of detecting various types of information obtained simultaneously by an OTDR method making use of Rayleigh scattered light.
The main line element and sensor element are connected to each other by a fusing technique which is widely applied, and therefore, the sensing optical fiber can easily be produced at a low cost.
To achieve the first technical object of the present invention, there is provided a sensing optical fiber comprising a main line element of optical fiber arranged as a light transmission line, and sensor elements of optical fiber connected by fusing to intermediate parts of the main line element. The sensor elements each have a core different in diameter from the core of the main line element and are relatively short in length.
In the light transmission line provided as one solution for achieving the first technical object of the invention, the sensor elements are formed as hetero-structures in the intermediate parts in the main line element of the light transmission line so as to allow a part of the light being propagated along the light transmission line to leak out to the clad of the optical fiber at the interface between the sensor element and the main line element. Enlargement of the change of the Rayleigh scattered light due to the ambient temperature, distortion or the like of the light transmission line makes it possible to increase the sensitivity of detecting the information. The main line element and sensor element are connected to each other by a fusing technique which is widely applied, and therefore, the sensing. optical fiber can easily be produced at a low cost. Furthermore, since the external shape of the light transmission line is made substantially uniform, the light transmission line can be handled by an ordinary fiber-laying technique.
Another solution for. achieving the first technical object of the invention is implemented by providing a sensing optical fiber, in which a number of sensor elements are. arranged at regular or irregular intervals in the intermediate parts of the main line element in the foregoing structure.
According to this solution, distribution of the changes of the Rayleigh scattered light propagated along the light transmission line can be perceived, and the sensitivity of detecting the information can be improved while widening the band width in which the information is detected.
Yet another solution for achieving the aforementioned first technical object of the invention is implemented by providing a sensing optical fiber, in which the optical fiber of the main line element is different in light transmission mode from the optical fiber in the foregoing structure.
According to this solution, since transformation of the transmission mode takes place in the sensor elements to increase the changes of the Rayleigh scattered light, which are caused due to the ambient temperature, distortion, liquid adhesion or the like of the light transmission line, the detection sensitivity of detecting the information can be increased.
A further solution for achieving the aforementioned first technical object of the invention is implemented by providing a sensing optical fiber which comprises a main line element of optical fiber arranged as an optical transmission line, and a relatively short sensor element connected by fusing to an intermediate part of the main line element and formed of material capable of passing light therethrough. The sensor element is different in refractive index from a core in the main line element.
According to this solution, the sensing optical fiber brings about the same function and effect as those described above, and besides, permits almost all of the light being transmitted through the main line element to leak out at the interface between the main line element and the sensor element, so that the changes of the Rayleigh scattered light can be extremely increased due to the ambient temperature, distortion or the like of the light transmission line. As a result, the light transmission loss brought about by the sensor element becomes large, but the detection sensitivity of detecting the information can be further increased.
A still further solution for achieving the aforementioned first technical object of the invention is implemented by providing a sensing optical fiber, in which a number of sensor elements are arranged at regular or irregular spacing intervals in the intermediate parts of the main line element in the sensing optical fiber.
According to this solution, the sensing optical fiber brings about the same function and effect as those described above.
The other solution for achieving the aforementioned first technical object of the invention is implemented by providing a sensing optical fiber, in which only one sensor element is inserted in the intermediate part of the main line element in the sensing optical fiber.
According to this solution, since the light transmission loss brought about by the sensor element becomes large, the light transmitted through the light transmission line can be intensively detected from the main line element with high efficiency.
One solution for achieving the second technical object of the invention is implemented by providing a sensor system, which comprises sensing optical fibers and a measuring instrument which detects information around the light transmission line incorporating the sensing optical fibers by using back-scattered light of the sensing optical fibers. The sensing optical fibers are formed of main line elements of optical fiber arranged as a light transmission line, and relatively short sensor elements are inserted in the intermediate parts of the light transmission line of the main line elements and each of the sensor elements has a core that is different in diameter from the core of the main line element. The sensor elements are different in number, spacing interval and diameter of core from one another. Also, an optical switch is placed between the respective sensing optical fibers and the measuring instrument so as to selectively switch the sensing optical fibers to be operated.
According to this solution, different types of information can be simultaneously transmitted through the sensing optical fibers having the same function and effect as those described above and selectively retrieved by. operating the optical switches so as to be detected by the measuring instrument, thereby to increase the detection sensitivity of detecting various types of information retrieved simultaneously.
Another solution for achieving the second technical object of the invention is implemented by providing a sensor system, which comprises sensing optical fibers and a measuring instrument which detects information around the light transmission line incorporating the sensing optical fibers by using back-scattered light of the sensing optical fibers. The sensing optical fibers are formed of main line elements of optical fiber arranged as a light transmission line, and relatively short sensor elements inserted in the intermediate parts of the light transmission line of the main line elements. The sensor elements are formed of material capable of passing light therethrough, and each has a core that is different in refractive index from a core in the main line element. The sensor elements are different in number, spacing interval and diameter of core from one another. Also, an optical switch is placed between the respective sensing optical fibers and the measuring instrument so as to selectively switch the sensing optical fibers to be operated.
According to this solution, different types of information can be simultaneously transmitted through the sensing optical fibers having the same function and effect as those described above and selectively retrieved by operating the optical switches so as to be detected by the measuring instrument, thereby increasing the detection sensitivity of detecting various types of information retrieved simultaneously.
Information such as deformation and temperature of an object to be inspected, a fire, flooding, vibration, breaking and angle can be detected by the sensing optical fiber or sensor system as mentioned above. The sensing optical fiber or sensor system as mentioned above may be used in cooperation with sensors of other types.